The present invention relates to damping force control type shock absorbers suitably used to damp vibrations of automobiles, for example.
In vehicles such as four-wheeled automobiles, a damping force control type hydraulic shock absorber is generally provided between a wheel-side member and a vehicle body-side member to damp vertical vibrations, etc. generated during running of the vehicle. This damping force control type hydraulic shock absorber uses an actuator to vary the orifice area through which a hydraulic fluid passes, thereby enabling damping force generated to be properly adjusted over a range of from low damping force to high damping force. Japanese Patent Application Publication No. Hei 07-332425 discloses an example of the actuator, in which a shutter serving as a variable orifice is rotated to control the damping force characteristics of a damping valve in a low piston speed region and also control the relief pressure of the damping valve (valve-opening pressure of the damping valve).
The above-described damping force control type hydraulic shock absorber disclosed in Japanese Patent Application Publication No. Hei 07-332425 uses a single variable orifice to control both the damping force characteristics of the damping valve in the low piston speed region and the relief pressure (valve-opening pressure) of the damping valve and therefore cannot control the damping force characteristics in the low piston speed region and the relief pressure independently of each other. Consequently, the degree of freedom for controlling the damping force characteristics is small, and it is difficult to properly adjust the damping force characteristics at each position over a range of from “soft” characteristics to “hard” characteristics. The difficulties in the adjustment of damping force characteristics may be compensated for by software control. Such a solution to the problem, however, will result in an increased cost.
There is a related-art technique having two variable orifices. With this technique, however, it is also difficult to properly adjust the damping force characteristics over the entire piston speed range of from a low piston speed region to a high piston speed region. In addition, because the flow rate of hydraulic fluid flowing through the variable orifices is high for the structural reason, the fluid force tends to increase. Therefore, a step-cut is likely to occur in the control of damping force characteristic.